In a large bowl, whisk butter and sugar together until creamed.
Add mashed bananas and extract, give it a quick mix. Note: it will look like the mixture has split but this is normal.
Add flour, rolled oats, cinnamon, baking soda and salt. Mix until well combined but do not over mix.
Place the mixture in the fridge and preheat the oven to 180C/356F. Line a baking sheet with baking paper. Note: the batter is easier to handle/shape when it's colder.
Roll 2 tbsp worth of batter into a ball and evenly spread onto the prepared baking sheet. Note: they don't spread much so place them 5 cm/2 inches.
Bake for 15-20 minutes or until they're t your liking. Note: the longer you bake them, the crispier they'll be. 15mins baking timw will yield a softer cookie.